The term “blind fastener” is often used to describe a fastener that can be fully installed from a single side of a structural assembly. Pop rivets and one-sided installation (OSI) bolts are two known types of blind fasteners. FIGS. 1A and 1B are partial cross-sectional views illustrating two stages in a method of installing a prior art blind fastener 100, and FIG. 1C is an end view of the blind fastener 100. Referring first to FIG. 1A, the blind fastener 100 includes a core bolt 110 extending through a passage 109 in a body 112. The core bolt 110 includes an exposed stem 118 and an externally threaded portion 115. The externally threaded portion 115 engages a nut 116. The nut 116 bears against a sleeve 114. As shown in FIG. 1C, the stem 118 of the core bolt 110 includes flats 111a and 111b, and the body 112 includes a head portion 119 (e.a., a countersunk head portion) having a plurality of recesses 113 (identified individually as recesses 113a-d).
Returning to FIG. 1A, to install the blind fastener 100, it is first inserted through a bore 104 in a first part 101 and a second part 102. A minimum distance D1 is required between the second part 102 and a third part 103 to provide clearance for the blind fastener 100 during installation. Next, prongs on a nose adapter of an installation tool (not shown) are engaged with the recesses 113 in the head portion 119 to prevent rotation of the body 112. A wrench adapter on the installation tool then engages the flats 111 on the core bolt stem 118 and rotates the core bolt 110 about its longitudinal axis. Rotation of the core bolt 110 in this manner causes the nut 116 to move toward the body 112. As this happens, the sleeve 114 flares out over the body 112 and presses against the second part 102 as shown in FIG. 1B. As the sleeve 114 is compressed, the core bolt stem 118 becomes harder and harder to turn until ultimately the stem 118 breaks off at a preset torque level. The foregoing installation procedure and related fastener details are described in the product specification entitled “OSI-BOLT™—HIGH STRENGTH FASTENER FOR PRIMARY STRUCTURE” provided by Monogram Aerospace Fasteners, Inc. of 3423 South Garfield Avenue, Los Angeles, Calif. 90022, and in U.S. Pat. Nos. 5,498,110 and 5,634,751, all of which are incorporated herein in their entireties by reference.
FIGS. 2A-2F are partial cross-sectional views illustrating a method for removing the blind fastener 100 from the first part 101 and the second part 102 in accordance with the prior art. Referring first to FIG. 2A, the method involves use of a drill tool 200 having a drill bit 204 rotatably disposed in an adapter 206. A plurality of prongs 202 (identified individually as a first prong 202a and a second prong 202b) extending outwardly from the adapter 206 engage the corresponding recesses 113 in the head portion 119 of the fastener body 112. The prongs 202 prevent the body 112 from rotating while the drill bit 204 removes a head portion 221 of the core bolt 110, as shown in FIG. 2B.
Referring next to FIG. 2C, after the head portion 221 of the core bolt 110 has been removed, a punch 230 is used to drive the remaining portion of the core bolt 110 out the backside of the fastener body 112. As shown in FIG. 2D, an increased backside clearance D2 is required between the second part 102 and the third part 103 to ensure that the core bolt 110 will fall clear of the body 112.
Referring next to FIG. 2E, a pilot tip 242 of an end mill 240 is inserted into the body 112, and a cutter portion 244 of the end mill 240 removes the head portion 119 of the body 112. As shown in FIG. 2F, a piloted rivet set 232 or other suitable tool is then inserted through the bore 104 and used to drive the body 112 out the backside of the bore 104.
As FIG. 1A illustrates, the minimum backside clearance D1 is all that is required to adequately install the blind fastener 100. As shown in FIG. 2D, however, removal of the blind fastener 100 by the method described above requires increasing the backside clearance to D2. Increasing the backside clearance from D1 to D2 solely for the purpose of fastener removal results in a larger structural assembly than would otherwise be required. In aircraft and other structures, the disadvantages of a larger structural assembly include an increase in structural weight and a decrease in usable space.